Microwave amplifiers, such as solid-state power amplifiers and traveling wave tube amplifiers, are widely used in communication systems for transmitting analog or digital data. A chronic problem with these amplifiers is that they exhibit nonlinear amplitude and phase transfer characteristics. These distortions are primary impediments to the reliable, spectrally-efficient transmission of data.
For an amplifier input signal having a modulated amplitude, as in many present-day communications systems, the nonlinear distortion is categorized as amplitude modulation to amplitude modulation (AM/AM) conversion and amplitude modulation to phase modulation (AM/PM) conversion. AM/AM conversion relates to the amplitude relationship at the amplifier input and output and can be defined as the change in gain with respect to a change in the input or output signal power. AM/PM conversion, on the other hand, relates to the amplitude and phase characteristics of the amplifier and can be defined as the change in the output signal phase relative to a change in the input or output signal power.
AM/AM conversion is a function of the power handling capability of the amplifier and can generally be lessened by reducing the drive level of the amplifier so that the output power is consideraly below saturation. This commonly used technique is known as "amplifier back-off". Unfortunately, this technique often does not adequately eliminate the AM/PM conversion. Consequently, in many system applications, AM/PM conversion is the major portion of the nonlinear distortion generated by microwave amplifiers.
Many prior art techniques which compensate for the nonlinear distortion generated by microwave amplifiers (see, for example, U.S. Pat. No. 3,755,754 to Putz and 4,283,684 to Satoh) have relied on circuits comprising signal splitters, amplifiers, phase shifters and signal combiners to generate a distortion correction signal. This correction signal is then added to the microwave amplifier input signal. A shortcoming of this technique is that it is often times not amenable to integrated circuit techniques and, therefore, the circuit design cannot be integrated onto a solid-state power amplifier substrate. As a result, the distortion compensation generated may not accurately track changes in the amplifier transfer characteristic with changes in amplifier operating temperature. In addition, the prior art circuits can be complex and expensive to implement.